The invention concerns a method of producing ingots or billets of metal—in particular steels and Ni- and Co-based alloys—by melting self-consuming electrodes in an electrically conductive slag bath using alternating current or direct current in a short, downwardly open water-cooled mold by way of which current contact with the slag bath can be made. The invention further concerns an apparatus for carrying out that method.
When producing remelt ingots in accordance with the method of electroslag remelting in stationary chill molds—but also in short sliding chill molds—it is usual, depending on the susceptibility to segregation of the remelted alloy, to set a melting rate in kilograms (kg) per hour, which in the case of round ingots is between 70% and 110% of the ingot diameter in millimeters (mm). In the case of ingot shapes which differ from a round cross-section such as square or flat formats, it is possible to operate with an equivalent diameter which is calculated from the periphery of the cross-section, divided by the number π (pi). The lower range is used in particular in relation to severely segregating alloys—such as tool steels or highly alloyed nickel-based alloys—, in relation to which the aim is to have a shallow metal sump for the avoidance of segregation phenomena. It is however scarcely possible to get below the value of 70% in the conventional electroslag remelting process as then the supply of power from the melting electrode into the slag bath has to be very greatly reduced, and that results in a low temperature of the slag bath and, as a further consequence, a poor, often grooved surface of the remelt ingot. With an excessively low supply of power to the slag bath a thick coating of slag is then also formed in many cases between the ingot and the mold, which in turn impedes the dissipation of heat from the surface of the ingot so that once again it is not possible to achieve the desired shallow molten bath sump. On the other hand however even in the case of steels and alloys which are less sensitive to segregation, it is not possible to exceed a value of 110% in the case of the conventional electroslag remelting process, referred to as the ESR method, as otherwise overheating of the slag bath together with the increased melting rate results in a molten bath sump which is unacceptably deep for remelt ingots, and thus an undesirably coarse ingot structure—linked to segregation phenomena. As can be readily seen from the foregoing, in the conventional ESR method in which the melting current is passed into the slag bath by way of the melting electrode and is removed again by way of the remelted ingot and the bottom plate, the slag bath temperature and the melting rate—and related thereto the sump depth and the nature of the surface—are closely linked together and cannot be monitored and controlled independently of each other and separately.
When producing remelt ingots of large diameter of 1000 mm and above, it is found that observing the above-indicated, desired low melting rates, particularly when using melting electrodes of large diameter, corresponding to 65 to 85% of the chill mold diameter, results in an excessively low slag bath temperature which then in turn results in the remelt ingot having a poor, often grooved surface. If in that case the supply of power to the slag bath is increased, that admittedly results in an improvement in the ingot surface, but at the same time that causes an increase in the melting rate above the admissible limit, which results in a deeper molten bath sump and disadvantageous hardening. That increase in the melting rate with an increased supply of power to the slag bath occurs for the reason that the melting electrode serves on the one hand to supply energy to the slag bath, but on the other hand it melts away correspondingly more quickly, the more the supply of energy to the slag bath is increased. The electrode then has to be suitably adjusted by movement into the slag bath at the speed at which it melts away. If the melting electrode were not adjusted in that way, it would melt away until just above the surface of the slag bath, whereby electrical contact and thus the supply of power to the slag bath would be interrupted. The remelting procedure would thus come to a stop.
Another way of increasing the slag bath temperature is that of remelting electrodes of smaller diameter. In that case the end face of the electrode, which dips into the slag bath, is smaller so that a comparatively hotter slag bath is required in order to achieve the desired melting rate. Admittedly, in many cases it is possible in that way to achieve an improvement in the surface of the ingot, but the use of electrodes of small diameter results in an increased concentration of heat in the center of the ingot, which can result in a sump which is depressed in a V-shape, with an increased tendency to segregation.
The cause of all the above-indicated difficulties is the fact that on the one hand the melting rate of the electrode is controlled by the energy which is fed to the slag bath by way of the electrode, and on the other hand it is precisely that feed of energy that must also be sufficient to keep the molten bath sump sufficiently fluid as far as the edge thereof and reliably to prevent a temporary progression of hardening beyond the meniscus of the molten bath sump. More specifically, if an excessively low temperature of the slag bath temporarily causes such a progression of hardening beyond the meniscus, that results in the formation of a grooved surface which is detrimental in terms of further processing of the ingots.
The present applicants' EP 786 521 B1 discloses a method of electroslag remelting, in which higher melting rates are set by melting electrodes of comparatively large diameter, than in the conventional electroslag remelting procedure. In the described method the return of a part of the melting current can be implemented by way of current-conducting elements which are installed in the wall of the chill mold. The arrangement results in a distribution of the return currents in inverse proportion to the total resistances of the conductor loops used.
In consideration of those factors the inventor set himself the aim of being able to control the melting rate of the electrode independently of the temperature of the slag bath and at the same time to ensure a good ingot surface.